The use of disc drive systems are currently moving beyond computers into other devices including digital cameras, digital video recorders (DVR), laser printers, photo copiers and personal music players. Disc drive memory systems are used for storage of digital information that can be recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
Discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. One of the bearings is located near the top of the spindle and the other near the bottom. The bearings permit rotational movement between the shaft and the hub, while maintaining alignment of the spindle to the shaft.
In a hydrodynamic bearing system, a lubricating fluid (gas or liquid) serves as the media to create pressure between a stationary base or housing and a rotating spindle or rotating hub. The dimensions of the gap between the rotating component and the stationary component of the motor must be tightly controlled to obtain good dynamic performance. The dynamic performance of a hydrodynamic motor is a function of the gap since gap pressure affects dynamic performance, and hydrodynamic and hydrostatic bearings utilize pressures. That is, a hydrodynamic bearing is a self-pumping bearing that generates a pressure internally to maintain a fluid film separation. A hydrostatic bearing requires an external pressurized fluid source to maintain the fluid separation.
Metal sections of the hydro bearing system are machined, making it difficult to obtain a gap with uniform or specified dimensions in a repeatable fashion and resulting in variations in the manufacturing process. The tight control required to produce small dimensions of the gap (in some applications 2 or 3 microns between the adjacent surfaces of a stationary shaft and rotating sleeve) makes precision machining bearing components difficult and costly. Precision machining is especially expensive when utilized to create a uniform surface on both a shaft and a sleeve. A bearing gap, in particular sections, should remain uniform and constant. When the bearing gap varies, nonrepetitive runout (NRRO), as well as other bearing performances are effected. Coating the bearing gap surfaces (i.e. shaft or sleeve surface) using a conventional sputtering process is unsatisfactory and inadequate since a coating thickness variation, a taper, and a variable bearing gap results. Further, there is a trend to decrease the aspect ratio (depth to width ratio) in sleeves. Moreover, there is a trend to evermore decrease bearing clearances to achieve greater recording densities. Additionally, gap variations may be specified in design, making the manufacturing process even more difficult.
Furthermore, it has become essential to select suitable material pairs that ensure negligible wear during operation of the motor. This is especially true in the case of mobile applications that must be shock-resistant, under both operating and non-operating conditions, and where precision parts are essential and gap tolerance is tight.